Radiotherapy devices include, for example, devices that enable dynamic conformal arc to irradiate therapeutic radiation intensively on an affected part by irradiating the therapeutic radiation while adjusting an irradiation field corresponding to a shape of the affected part using a multi leaf collimator (MLC) from a plurality of directions (Patent Literature 1). Patent Literature 1 further describes a volumetric modulated arc therapy (VMAT) in which the radiation is irradiated while a gantry is rotated.